I've just come back from a trip to the Michigan Dunes and while I was there I caught minnows, suckers, and surprisingly, Killies. They have vertical green/blue bars running down their body. I have what appears to be 2 males and 2 females in a 10g that was supposed to be a breeder tank. The bottom is covered with hornwort that is weighted, and the temp is around 72. I'm going to remove the heater later, but I'm trying not to stress them too much now.

When I tried to ID them, the best I could come up with was a Banded Killi or a Plains Killi. The pictures on Google don't serve my guys any justice as they are almost iridescent right now. They survived a 2.5 hour ride home at around 35 degrees Fahrenheit They were sitting 'dead' at the bottom of the bag for 2 minutes before they perked up in warmer water.

I'll try to get pictures but my camera is horrible._________________Trying to spawn Barbs and Clowns
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Monstrosity is correct - banded killies are the only topminnows found in Lake Michigan.
Was the water at 35 degrees. I hope you acclimated them very slowly if your tank is at 72, because that is very very hard on cool-water fish. Definitely remove the heater. They are jumpers, so make sure you have a tight fitting lid. Otherwise, they are good, un-demanding fish. Good luck with them!

Ya it took them about a half hour to acclimate. The heater is gone. I added a top last night... woke up to find the top drenched in water I'll give them some bloodworms tonight and then see if they take to pellets. All they do is hide in the hornwort all day _________________Trying to spawn Barbs and Clowns
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By filter I'm sure you meant heater. Regardless of my acclimation techniques, all are fine now. I chose the prettiest male and biggest female to keep in the 10g. The other two were put in my pond outside where they can do some good eating mosquitoes.

I'm not liking how they just dive down into the hornwort, but I'm not removing it because it is still my breeder tank. I'll try instigating some breeding, see what I can get._________________Trying to spawn Barbs and Clowns
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They are not terribly hard to breed. You need more females than males (1M:4F ratio), to get decent success in aquaria. They will also do this in your pond for you fairly easily if you have plenty of plants or long roots hanging down. I know of a decent write-up from a guy who bred banded killies last fall. I'll see if I can scrounge it up..

And yes, when you have that drastic of a water temp, it is best to have a hour-long acclimation period. When I collect in wintertime, I use a airhose (like what air-pumps/bubblers use) to siphon tank water down into my cooler or bucket where my recently collected fish are. This is a very slow (but not quite a drip-acclimation) to help bring them to tank temp. Just be sure to peak at your bucket or cooler to make sure it doesn't over flow, hehe.

Well if we collect anymore they will probably go to the pond. Everyone but the male hid all day in the hornwort so we assumed he was bullying them... or something. I just thought they were an interesting fish so I kept a few. They will most likely be periodically evicted to make room for spawning Cherry Barbs. I guess eventually they will end up in the pond. If you could find that article that would be awesome. I'm always up for a challenge with a 1:1 ratio._________________Trying to spawn Barbs and Clowns
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They will be the best mosquito eating fish you've come across - I promise.
Here is a link to a quick write-up that a NANFA member / fish dealer wrote last year. Not too terribly detailed, but may suffice for your trials.
Western Banded Killifish Captive Spawn in Aquarium